1. Technical Field
The present invention relates to an apparatus for steering the rear wheels of a vehicle. In particular, the present invention relates to an apparatus for selectively locking the rear wheels to prevent turning of the rear wheels.
2. Description of Prior Art
Vehicles having steerable front wheels and steerable rear wheels are known. A steering system for such a vehicle is disclosed in U.S. Pat. No. 4,645,025. The steering system disclosed in U.S. Pat. No. 4,645,025 includes a steering gear for turning the rear wheels in response to steering the front wheels. The steering gear includes a housing supporting a rack member for axial movement. A pinion has teeth in meshing engagement with teeth on the rack member. The pinion is rotated by an electric motor to axially move the rack member to thereby turn the rear wheels. The direction and amount of movement of the rack member is determined by a controller and is a function of vehicle speed and the direction and amount of steering of the front wheels. In the event of a failure of the electric motor, or the like, the rack member is spring biased to a position in which the rear wheels are in a straight-ahead position. There is no positive locking of the rear wheels in the straight-ahead position.
U.S. Pat. No. 4,671,523 discloses a steering system for steering the front wheels and rear wheels of a vehicle. The steering system includes a steering gear for the rear wheels. The steering gear for the rear wheels includes an electric motor which drives a pinion connected with a rack member. When the rack member moves, the rear wheels turn. The rack member has a notch. A locking member is spring biased into the notch to block movement of the rack member and maintain the rear wheels in a straight-ahead position. Pressurized fluid moves the locking member to disengage the rack member.
If a vehicle having the steering system of U.S. Pat. No. 4,671,523 is travelling straight on a laterally angled surface, the rack member will apply a relatively large force to the locking member. Substantial frictional resistance to movement of the locking member to its disengaged position will result. Thus, a relatively large force must be applied to the locking member to disengage the locking member from the rack member. If an electrically actuatable solenoid or motor was used to move the locking member, the size of the solenoid or motor would have to be so great as to make it impractical. Also, when the locking member is in its locking position, it is subjected to relatively high forces applied to it through the rack member from the road. Moreover, the locking member could be subjected to high forces in the event of a system failure. For example, if the electric motor is energized and the locking member is not disengaged from the rack member, the forces applied to the locking member could break the locking member.